1. Field of the Invention
The present invention relates, generally, to heat engines. More particularly, the invention relates to Stirling cycle heat engines with a cylinder containing a working fluid and a piston moving therein.
2. Background Information.
The maximum Stirling engine efficiency is related to the Carnot efficiency which is governed by the ratio of maximum working fluid temperature relative to the minimum fluid temperature. Improvements in technologies which increase the margin between the two temperature extremes is beneficial in terms of total cycle efficiency. The lower working fluid temperature is typically governed by the surrounding air or water temperature; which is used as a cooling source. The main area of improvements result from an increase in the maximum working temperature. The maximum temperature is governed by the materials which are used for typical Stirling engines. The materials, typically high strength Stainless Steel alloys, are exposed to both high temperature and high pressure. The high pressure is due to the Stirling engines requirement of obtaining useful power output for a given engine size. Stirling engines can operate between 50 to 200 atmospheres internal pressure; for high performance engines.
Since Stirling engines are closed cycle engines, heat must travel through the container materials to get into the working fluid. These materials typically are made as thin as possible to maximize the heat transfer rates. The combination of high pressures and temperatures has limited Stirling engine maximum temperatures to around 800xc2x0 C. Ceramic materials have been investigated as a technique to allow higher temperatures, however their brittleness and high cost have made them difficult to implement.
U.S. Pat. No. 5,611,201, to Houtman, shows an advanced Stirling engine based on Stainless Steel technology. This engine has the high temperature components exposed to the large pressure differential which limits the maximum temperature to the 800xc2x0 C. range. U.S. Pat. No. 5,388,410, to Momose et al., shows a series of tubes, labeled part number 22 a through d, exposed to the high temperatures and pressures. The maximum temperature is limited by the combined effects of the temperature and pressure on the heating tubes. U.S. Pat. No. 5,383,334 to Kaminishizono et al, again shows heater tubes, labeled part number 18, which are exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,433,078, to Shin, also shows the heater tubes, labeled part number 1, exposed to the large temperature and pressure differentials. U.S. Pat. No. 5,555,729, to Momose et al., uses a flattened tube geometry for the heater tubes, labeled part number 15, but is still exposed to the large temperature and pressure differential. The flat sides of the tube add additional stresses to the tubing walls. U.S. Pat. No. 5,074,114, to Meijer et al., also shows the heater pipes exposed to high temperatures and pressures.
The Stirling engine disclosed in the inventor""s U.S. Pat. No. 6,041,598 overcomes the limitations and shortcomings of the above prior art by providing a dual shell pressure chamber. An inner shell surrounds the heat transfer tubing and the regenerator. The portion surrounding the heat transfer tubing contains a thermally conductive liquid metal to facilitate heat transfer from a heat source to the heat transfer tubing and also to transmit external pressure to the heat transfer tubing. An outer shell that acts as a pressure vessel surrounds the inner shell and contains a thermally insulating liquid between the inner and outer shells. Pressure of the working fluid as it flows through the regenerator is transmitted through the inner shell to the insulating liquid and back across the inner shell to the liquid metal surrounding the heat transfer tubing. This system tends to balance the pressure across the heat transfer tubing and the inner shell, thereby allowing the engine to operate with the working fluid at a high pressure to generate significant power while keeping the wall of the heat transfer tubing thin to facilitate heat transfer through it.
A component in all the Stirling engines which is critical to the maximum performance is the regenerator. This device must heat and cool the working fluid for each cycle of the engine which may be 20 to 100 times per second. Past regenerators typically have been made of fine mesh screens very densely packed into layers which are 100""s of screens thick. The fine screens and multiple layers are required to meet the high heat transfer rate requirements. These screen regenerators have significant pressure drop as the working fluid, typically Helium, Hydrogen, or air moves through the mesh at high speeds. The performance of the Stirling engine is thusly limited by the use of mesh screens. Other regenerators have used random metal matrix materials and felt with steel filaments oriented primarily perpendicular to the flow direction. Again there are significant pressure drops associated with use of those materials.
Pressure drop through a regenerator can be reduced by using a regenerator that has parallel-duct flow channels, but the surface area must be very large to achieve the necessary heat transfer. U.S.. Pat. No. 5,388,410, to Momose et al., shows a mesh regenerator (No. 25) located inside the heating and cooling tubes, and an improved regenerator (No. 26) that uses a series of small annular pipes placed inside the heater pipe. The maximum heat transfer rate is limited by the minimum pipe diameter. The small tubes also touch each other on their exterior which blocks the working fluid flow.
For very small Stirling engines a single annular slot has been used with success. The slot reduces the pressure drop but its effectiveness is limited by the amount of surface area in a single slot regenerator.
The present invention provides a regenerator for a Stirling engine which overcomes the limitations and shortcomings of the prior art.
The present invention provides a multi-channel thermal regenerator for a heat engine comprising a coiled annulus of thin high-temperature sheet material coiled so that individual coils are in a spaced substantially parallel arrangement in an annular configuration. Adjacent coils form channels for moving fluid therethrough such that heat is transferred between a fluid moving through the channels and the coils with a minimum pressure drop in the fluid.
In one embodiment the sheet material is made of metal, preferably stainless steel approximately 0.002 inches thick and embossed, preferably in a rectangular array in order to leave open channels, with protruding dimples that protrude approximately 0.008 inches from the surface to space the coils.
In another embodiment the sheet material is made of carbon having a plurality of fibers oriented generally circumferentially around the coiled annulus providing a higher thermal conductivity in a direction along the fibers. A ceramic string is preferably woven between the coils radially across the annulus at a plurality of locations around the annulus to space the coils.
The coiled annulus is preferably contained in a cartridge having an inner sleeve and an outer sleeve in a spaced parallel arrangement from the inner sleeve with the coiled annulus being disposed therebetween. A cage base connects the inner sleeve and outer sleeve at one end of both sleeves. The cage base has a plurality of bars adjacent an end of the coiled annulus for spacing the end of the coiled annulus from an engine structure against which the cartridge is installed.
The features, benefits and objects of this invention will become clear to those skilled in the art by reference to the following description, claims and drawings.